Polyaminocarboxylate ligands, and the chelates derived therefrom, have been widely used in medical diagnosis and therapy, such as for example in the field of Magnetic Resonance Imaging (MRI). Macrocyclic chelating agents, such as DOTA macrocyclic chelating agents, form particularly stable chelates with contrast-generating paramagnetic metal ions, and thus are suitable carriers for these metal ions. The gadolinium-DOTA chelate (Dotarem®) is one commercially available MRI agent. Radionuclide chelates, such as 177Lu-DOTA and 90Y-DOTA, conjugated to bioactive peptides have also been used as radioscintigraphic imaging and radiotherapeutic agents.
Lack of efficient and cost effective processes for the synthesis of polyazamacrocyclic ligands has been an obstacle toward widespread use of these types of ligands and associated chelates. Several synthetic routes for the preparation of DOTA are known. For instance, EP 232751 A (by Tweedle) and EP 292689 A (by Tweedle) disclose DOTA preparation by diamine:diamine or triamine:monoamine cyclic condensation.
A key intermediate in these procedures is 1,4,7,10-tetraazacyclododecane. Hansen, et al. (Hansen and Burg. Journal of Heterocyclic Chemistry 1968, 305) disclose that 1,4,7,10-tetra(benzyl)-1,4,7,10-tetraazacyclododecane can be produced by cylco-tetramerization of N-benzylaziridine according to the following reaction scheme:
Notably, however, Hansen discloses that the cylco-tetramerization process shown above is unique to the N-benzylaziridine substrate, as only high molecular weight polymers, and not macrocycles, were generated when aziridine, N-methylaziridine, N-phenylaziridine and N-(β-hydroxymethyl) aziridine substrates were used.
Building upon Hansen, WO9628420A2[1] (WO '420 by Messerle) and U.S. Pat. No. 5,744,616 (U.S. '616, by Petrov) disclose a process for the preparation of DOTA and Dotarem® from N-benzylaziridine substrate according to the below reaction scheme, where WO '420 discloses isolation of each intermediate before proceeding to the subsequent process step and U.S. '616 discloses carrying each intermediate forward in solution.
However, WO '420 and U.S. '616 disclose tetra-benzyl cyclized intermediate yields of about 28% and about 58%, respectively.
A need therefore exists for improved and simplified processes for the preparation of macrocyclic polyazacaboxylate ligands, and more specifically 1,4,7,10-tetraazacyclododecane derivatives, such as DOTA, DOTA-chelates, and derivatives thereof, in high yield and purity.